1. Field of the Invention
The present invention relates to securing optical elements, more particularly, to applying an optically inert, light absorbing coating on an optical element for the protection of adhesives used to secure optics from ultra-violet light.
2. Related Art
The secure positioning of optical elements within an optical system is crucial, particularly in very high precision optical systems, such as those used in photolithography. Optical system developers use several approaches to securely position optical elements within a system, including clamping an optical element in place, encasing an optical element entirely within an enclosure, and using an adhesive to hold an optical element in place. The current invention relates to approaches using an adhesive to hold an optical element in place. The term adhesive as used herein refers to any chemical compound used to secure an optical element, including but not limited to glues, epoxies, and cements.
The use of an adhesive to hold an optical element in place provides a cost-effective means to securely position an optical element. Additionally, the use of adhesives to secure an optical element in place imparts less mechanical stress on the optical element than a purely mechanical mount. Typically, an adhesive is applied to an optical element and the optical element positioned within a mount or other fixture. The adhesive may be applied to either an edge or a portion of the polished surface of an optical element depending on the desired orientation of the optical element. The optical element is held in place while the adhesive is permitted to dry. Upon drying, the adhesive between the optical element and the mount provides a solid bond that holds the optical element securely in place.
Unfortunately, scattered light from light passing through an optical element may photochemically degrade the adhesive and cause the bond to weaken over time. Within optical elements that allow light to pass through their polished surfaces, a portion of the scattered light is internally reflected and propagates to points where an adhesive bonds with the optical element. In the case where an adhesive is used to hold the optical element in place, the scattered light impinges on the adhesive. The light impinging on the adhesive causes the adhesive to break down, and the bond between the optical element and the mount begins to weaken. As the integrity of the bond weakens, the optical element may move or even become dislodged. Neither situation is acceptable, particularly in very high precision optical systems.
In addition, many adhesives undergo out-gassing when they are impinged upon by ultra-violet light. These out-gasses may have the deleterious impact of creating a thin residue on the polished optical surfaces that impairs the functioning of the optical system.
Placing a light absorbing coating on points of an optical element where adhesive will be applied prior to affixing it with an adhesive addresses the above problems by reducing the amount of light that impinges upon the adhesive. There are numerous methods, such as chemical vapor deposition and ion beam sputter for applying a coating to an optical element. Systems using these approaches principally are designed to very precisely coat the polished optical surfaces. As a result, they are very expensive, require extensive set-up time, and are particularly poorly suited for applying a coating to an edge of an optical element.